Common ion effect to assist LM separation

ABSTRACT

The instant invention relates to an improvement in the liquid membrane process for removing soluble materials from solution. More specifically, in a process wherein soluble materials are removed from solution by contacting said solution with an emulsion, the external phase of which is immiscible with said solution and permeable to said soluble materials and the internal phase contains a reactant which reacts with said soluble material and converts said soluble material to a material which is insoluble and thus trapped in the internal phase, the improvement which comprises maintaining the concentration of said reactant in the internal phase of the emulsion at a level at which the concentration of said soluble material in the internal phase of the emulsion is less than the concentration to which it is desirable to lower said soluble material in said solution. Preferably the solution is aqueous and said soluble material is a salt. In a preferred embodiment of the instant invention the concentration of fluoride ion in an aqueous solution is lowered to less than 6 ppm by contacting said solution with an emulsion containing a soluble calcium salt in the internal phase. The concentration of calcium in said internal phase is maintained at a level sufficient to provide a concentration of less than 6 ppm soluble fluoride ion in said internal phase.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Ser. No. 404,716filed Oct. 9, 1973 in the names of Norman N. Li, Robert P. Cahn and AdamL. Shrier which is division of U.S. Ser. No. 174,990 filed Aug. 5, 1971(now U.S. Pat. No. 3,779,907 granted Dec. 18, 1973), which is acontinuation-in-part of .[.Ser. No. 99,267, filed Dec. 17, 1970 (nowU.S. Pat. No. 3,719,590, granted Mar. 6, 1973).]. .Iadd.Ser. No. 36,686,filed May 12, 1970 (now U.S. Pat. No. 3,637,488, granted Jan. 25, 1972).Iaddend..[.and .]. .Iadd.which .Iaddend.is a continuation-in-part ofSer. No. 28,094 filed Apr. 13, 1970 (now U.S. Pat. No. 3,617,546,granted Nov. 2, 1971).

FIELD OF THE INVENTION

The instant invention relates to an improvement in the liquid membraneprocess for removing soluble materials from solution. More specifically,in a process wherein soluble materials are removed from solution bycontacting said solution with an emulsion, the external phase of whichis immiscible with said solution and permeable to said soluble materialsand the internal phase contains a reactant which reacts with saidsoluble material and converts said soluble material to a material whichis insoluble and thus trapped in the internal phase, the improvementwhich comprises maintaining the concentration of said reactant in theinternal phase of the emulsion at a level at which the concentration ofsaid soluble material in the internal phase of the emulsion is less thanthe concentration to which it is desirable to lower said solublematerial in said solution. Preferably the solution is aqueous and saidsoluble material is a salt. In a preferred embodiment of the instantinvention the concentration of fluoride ion in an aqueous solution islowered to less than 6 ppm by contacting said solution with an emulsioncontaining a soluble calcium salt in the internal phase. Theconcentration of calcium in said internal phase is maintained at a levelsufficient to provide a concentration of less than 6 ppm solublefluoride ion in said internal phase.

BACKGROUND OF THE PRIOR ART

Liquid membrane processes are known for the removal of soluble materialsfrom solution, for example, see U.S. Pats. Nos. 3,389,078 and 3,410,794,wherein hydrocarbons are separated from solution by means of a liquidmembrane technique which comprises contacting saidhydrocarbon-containing solution with an oil-in-water emulsion, said oilbeing a solvent for said hydrocarbon and said hydrocarbon being capableof permeating through the water phase of said emulsion. In theseprocesses permeation of the hydrocarbon from solution into the internalphase of the emulsion continues until the concentration of saidhydrocarbon in the solution and in the internal phase is equal. At thispoint there no longer exists a driving force for the hydrocarbon topermeate through the external phase of the emulsion into the internalphase. Thus processes of this nature are limited by the solubilityrequirements of the internal phase of the emulsion.

In U.S. Pat. No. 3,779,907 dissolved materials are removed from aqueoussolution by the reverse method of contacting a water-in-oil emulsionwith the solution and allowing permeation of said dissolved materialsthrough the nonaqueous external phase into the aqueous internal phase ofthe emulsion. In this patent, however, the dissolved species continue topermeate through the oil phase, i.e. external phase of the emulsion, dueto the fact that in the internal phase of the emulsion a reactant ispresent which converts the dissolved species to nonpermeable materials.In effect, the driving force is maintained by converting the permeabledissolved species into another form. This process, however, has alsobeen found to be limited by the fact that certain conversions in theinternal phase are not essentially complete, especially when the meansof conversion in the internal phase is precipitation. Certainprecipitating agents while effectively forming precipitates with thepermeable species, form precipitates which are themselves slightlysoluble. Thus, the permeable dissolved species is not completely removedfrom the aqueous solution but only removed down to a level approximatelyequal to the concentration of said permeable species soluble in saidinternal phase of the emulsion, i.e. the concentration of said permeablespecies due to the slight solubility of the precipitate.

It has now been found and is herein claimed as an invention that thisheretofore limit on the liquid membrane processes can be overcome bymaintaining the concentration of the precipitating agent at a levelwhereby due to the common ion effect the solubility of the permeablespecies in the internal phase is suppressed.

SUMMARY OF THE INSTANT INVENTION

The instant invention relates to a process for the removal of solublematerials from solution which comprises contacting said solution with anemulsion the external phase of which is immiscible with said solutionand yet permeable to said soluble material and the internal phasecomprising a reagent which will react with the permeable solublematerial and convert same into an insoluble precipitate, the improvementwhich comprises maintaining the concentration of said reagent at a levelwhereby the concentration of said permeable soluble material soluble inthe internal phase is suppressed by means of the common ion effect to alevel below the level to which it is desired to remove said solublematerial from said solution.

This invention is particularly applicable for removing dissolved salts,that is either the anion or the cation from aqueous solution. It isknown in the art that various materials can be precipitated from aqueoussolution. However, it is also known that the precipitates themselves areto a greater or less degree soluble in aqueous solutions. It is thislimitation which has heretofore allowed otherwise operable liquidmembrane processes to remove soluble salts only down to a level which isapproximately equal to the concentration of the soluble salt in theinternal phase of the liquid membrane emulsion due to the solubility ofthe precipitate in the internal phase. It is known that by means of thecommon ion effect that solubility of one or more of the anionic orcationic components of the water insoluble salt can be decreased. Forexample, due to the fact that the solubility product Ks=[A⁺ ][B⁻ ] isfixed, increasing the concentration of B⁻, i.e., increasing [B⁻ ] willdecrease [A⁺ ] i.e., lower the concentration of soluble A⁺. Thus, goodremoval of A can be obtained by maintaining a high concentration of Binside the membrane, i.e., in the internal phase of the emulsion.

The use of this technique will be exemplified below for the removal offluoride ion from water. It should be pointed out, however, that thismaterial is, for the purpose of example only and that those skilled inthe art will readily appreciate that this technology is applicable tothe removal of any dissolved compound from solution by means of liquidmembrane process wherein said dissolved compound is converted in theinternal phase of the emulsion into an insoluble precipitate by means ofa double decomposition reaction, preferably ionic ##STR1##

Here, either "A" or "B" are the dissolved species which are to beremoved, and by maintaining an excess of the other present in theinternal solution, the solubility of AB is suppressed.

Thus, if A is the specie to be removed, an excess of MB is maintained inthe internal solution, assuring a high concentration of [M⁺ ] and [B⁻ ]ions in the solution. As A diffuses across the membrane it will reactaccording to the equilibrium reaction, above, or simply

    [A.sup.+ ]+[B.sup.- ]⃡[AB]↓

which is set by the above mentioned solubility equilibrium K_(s) =[A⁺][B⁻ ]. Since there is a large excess of B⁻, [A⁺ ] will be small andinward permeation of A will continue.

A similar argument holds when B is the specie to be removed, in whichcase an excess of AC is maintained in the internal solution.

An aqueous solution containing fluoride ion, for example, sodium orammonium floride dissolved therein, is contacted with a water-in-oilemulsion under conditions whereby the emulsion is dispersed as drops insaid aqueous solution. The emulsion, being of the water-in-oil type, isimmiscible with said aqueous solution, thus, the external phase of theemulsion exists as an oil membrane surrounding droplets of the internalphase of said emulsion. The external phase of the water-in-oil emulsionis tailored by means known in the art (see the above liquid membranepatents and patent applications which are herein incorporated byreference) to be permeable to fluoride ion. For example, emulsions maybe made up by emulsifying the aqueous internal phase in an oil phasecomprising a polyamine derivative¹ as a membrane strengthening agent,Alamine 336 or Aliquot 336 as the transferring agent for the anionicfluoride, and SIOON² as the solvent to dissolve all the above ##STR2##

The transfer of fluoride will be based on an ion-exchange mechanism.This means that the transferring agent in the membrane phase willcomplex with fluoride ions in the solution and move them across themembrane to the other side, where the transferring agent will releasethe fluoride ions and complex with another anionic species, such as Cl⁻and transfer it back to the solution. For further discussion of thismechanism, see copending U.S. Pat. Application Ser. No. 669,706, filedon Mar. 23, 1976 in the names of the instant inventors. Of course, inthe internal phase of the emulsion, that is the aqueous phase utilizedto make up the water-in-oil emulsion, a reagent which will react withfluoride ion and form a precipitate is incorporated. In general, awater-soluble calcium salt is utilized in the aqueous phase when it isdesirable to remove fluoride, for example, calcium halides, especiallycalcium chloride, calcium nitrate, calcium acetate, calcium hydroide andcombinations of these may be incorporated at a level of from 0.1 to 50weight %. It is noted that it is critical in the practice of the instantinvention that sufficient calcium be incorporated in the internal phaseto yield an excess so that by the common ion effect solubility of thecalcium fluoride precipitate will be suppressed. In general, theconcentration of calcium ion in the internal phase should be about 0.05wt. % or more, preferably 0.5-10%. It is noted that the amount ofcalcium ion necessary is dependent on how much fluoride ion must beremoved from the aqueous solution. This calculation can readily beeffected by anyone skilled in the art and is illustrated in thefollowing table.

                  Table I                                                         ______________________________________                                        Fluoride Concentration in Equilibrium with                                    Excess Dissolved CaCl.sub.2 at 25° C.                                  Excess CaCl.sub.2  Equilibrium Fluoride                                       Concentration Wt. %                                                                              Concentration, ppm                                         ______________________________________                                        0                  8.2                                                        0.01               4.0                                                        0.1                1.27                                                       1                  0.40                                                       2                  0.28                                                       4                  0.20                                                       10                 0.12                                                       20                 0.08                                                       30                 0.06                                                       40                 0.05                                                       ______________________________________                                    

Note that the above table shows the effect of excess CaCl₂ over andabove the quantity required to precipitate the fluoride permeating intothe emulsion. Thus, if it is desired to spend 80% of a 10 weight % CaCl₂emulsion, then 8 weight % of CaCl₂ will be converted into a 5.6 weight %CaF₂ precipitate, but 2 weight % of CaCl₂ will remain as excessdissolved salt, which allows the fluoride level in the remainingsolution to rise no higher than 0.28 ppm. Therefore, the fluoride levelin the solution being treated can be pulled down much lower than the 8.2ppm indicated by the plain solubility product of CaF₂ alone.

Since the excess CaCl₂ is in the internal phase of the liquid membraneemulsion, it does not contaminate the solution being treated. On theother hand, if the low level of 0.28 ppm F⁻ was to be obtained bystraight precipitation via addition of CaCl² directly to the solution, a2% residual excess of CaCl₂ would be left in the solution after thetreat. This is a highly undesirable contaminant in the effluent. Inanother example, an aqueous solution comprising 1000 ppm fluoride ion iscontacted with a water-in-oil emulsion containing 10 weight % calciumdichloride based on the aqueous phase contained in the emulsion. Thisemulsion is suitable for contacting approximately 37 volumes of saidaqueous solution per volume of contained CaCl₂ solution. In this examplewhich is carried out at room temperature it is known that calciumfluoride in stoichiometric amounts will yield 8 ppm fluoride. Thuswithout an excess of calcium in the internal phase of the emulsion thefluoride level of the aqueous solution may be reduced only to this levelin a continuous operation where fresh emulsion is continuously made upand added to the system, and spent emulsion is continuously withdrawn.In certain applications it is desirable to remove fluoride to a lowerlevel. An excess of calcium ion is thus placed in the internal phase ofthe emulsion. Thus, by starting with 14 weight % CaCl₂ in the internalaqueous phase, and again treating 37 volumes of said aqueous solutioncontaining 1000 ppm fluoride per volume of CaCl₂ solution encapsulatedin the emulsion, the spent emulsion will contain 4 weight % excessCaCl₂. This allows, as is obvious from Table I, removal of fluoride fromthe aqueous solution to a level of 0.2 ppm.

Again, it is noted that the addition of calcium ion directly to theaqueous solution would yield essentially the same result in thatfluoride ion would be removed. However, the calcium ion being used inexcess would yield an undesirable degree of contamination in the treatedwater. Thus additional treatment would be required.

The emulsions useful in removing the water-soluble salts from aqueoussolution are generally described in U.S. Pat. No. 3,779,907 hereinincorporated by reference above. It is noted that the precipitatingreagent which is to be used in the instant process will be selected onthe basis of what is known in the art about precipitating of dissolvedmetal salt components. For example, many metals will precipitate fromaqueous solution by combination with sulfide ion. Sulfide ion thus maybe used in the internal phase of the emulsion. Again, however, if it isdesired to remove the metal to a point below that which is present dueto the solubility product of the sulfide precipitate excess sulfide mustbe used, for example in the form of excess sodium sulfide.

The following are specific embodiments of the instant invention.

EXAMPLE 1 Fluoride Removal by Precipitation as CaF₂

The solubility of CaF₂ in water is about 0.0016- 0.0017 g/100 cc at18°-25° C. This fixes the attainable fluoride ion level whenprecipitating CaF₂ with the essentially stoichiometric amount of calciumat 8 ppm. The use of a large excess of precipitating calcium ion in thetreated waste water to decrease the fluoride ions contribution to thesolubility product of the CaF₂ precipitate is undesirable as it leads toexcessive dissolved calcium salt, which would therefore requireadditional treatment.

Consequently, it is particularly advantageous to use a large excess ofcalcium ion on the inside of the liquid membrane emulsion. This can bedone by using not only Ca(OH)₂ which is only sparingly soluble but willneutralize the fluoride permeating across as it precipitates as CaF₂,but also by adding a large excess of CaCl₂, say 10 wt. % on the internalsolution. As was discussed above, with stoichiometric Ca(OH)₂ present,the fluoride concentration inside the emulsion droplets cannot dropbelow 8 ppm, which sets a corresponding limit to the attainableF-cleanup on the waste water being treated. Incorporation of the above10% CaCl₂ lowers the achievable F-concentration to 0.06 ppm.

EXAMPLE 2 Heavy Metal Removal by Sulfide Precipitation

Similarly to the CaF₂ case discussed above, heavy metals can be removedbut require a precipitating agent. Sulfide is very effective, but toreach the low residual metal level often required would call forexcessive sulfide concentration in the waste water (which normallyshould have very few ppm of sulfide in it). Again, a high common ionconcentration inside the membrane can easily be established by means ofNa₂ S dissolved in the internal droplets (preferably in the presence offree NaOH to minimize H₂ S permeation out of the membrane). This excesssulfide assures very complete removal, via precipitation and permeation,of the heavy metals in the waste water stream.

EXAMPLE 3 Removal of Heavy Metals as Hydroxide

Similar to the sulfide above, hydroxide can be used. Many metals forminsoluble hydroxides, i.e., Fe(OH)₃, Mn(OH)₂, Cr(OH)₃ which can be verycompletely precipitated in a high pH medium, where [OH⁻ ] is high.Again, this can be achieved inside of a liquid membrane emulsion withoutaffecting the pH of the bulk water phase being treated.

What is claimed is:
 1. In a process for removing a soluble material fromsolution by contacting said solution with an emulsion, the externalphase of which is immiscible with said solution and permeable to saidsoluble material and the internal phase contains a reagent which reactswith said permeable soluble material to form an insoluble precipitate,which is trapped in said internal phase, the improvement which comprisesmaintaining the concentration of said reagent in the internal phase ofthe emulsion at a level at which the concentration of said solublematerial in the internal phase of the emulsion is suppressed by means ofthe common ion effect to a level below the level to which it is desiredto remove said soluble material from said solution.
 2. The process ofclaim 1 wherein said solution is aqueous.
 3. The process of claim 2wherein said soluble material is a salt.
 4. The process of claim 3wherein said permeable soluble material is fluoride ion.
 5. The processof claim 4 wherein said internal phase comprises a water soluble calciumsalt.
 6. The process of claim 5 wherein said internal phase comprises0.5 to 10% by weight calcium ion.
 7. The process of claim 6 wherein saidwater soluble calcium salt is selected from the group consisting ofcalcium chloride, calcium nitrate, calcium hydroxide, calcium acetateand mixtures thereof.
 8. The process of claim 3 wherein said permeablesoluble material is a heavy metal which forms an insoluble precipitatewith sulfide ion.
 9. The process of claim 8 wherein said internal phasecomprises Na₂ S.
 10. The process of claim 3 wherein said permeablesoluble material is selected from the group consisting of Mn⁺⁺, Cr⁺⁺⁺and Fe⁺⁺⁺ and said reagent is hydroxide ion.